I believe I mentioned this before somewhere but (probably just in the CB, actually):

BEGIN { $sep= '' }
if(  '' eq $sep  &&  1 < @F  ) {
    ( $sep )= /\Q$F[0]\E(.*?)\Q$F[1]\E/;
}
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Sadly, although join(undef,@F) needn't emit a warning when @F contains fewer than 2 items, it does.

Yes, it still isn't perfect (but I hope it helps some). I'm sure you know where to apply the patch. $^A doesn't appear to be used (for "auto-split separator"), last I checked.

Update: I was trying to think how to wrap this nicely into a module and also that the separator needn't be the same at each point, when I decided the way to do both would be to have the module provide a routine that replaces the old @F values with the new into $_ in-place.

my $pos= 0;
for my $f (  0..$#F  ) {
    pos($_)= $pos;
    die if ! m/\Q$F[$f]\E/g;
    $pos= pos($_) + length($G[$f]) - length($F[$f]);
    substr( $_, $-[0], $+[0]-$-[0], $G[$f] );
}
pos($_)= 0;
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And I'd also like a module that ties up this trick for avoiding using $& (and capturing) that is also more useful (and faster) than either so I don't have to look up the formula each time. (: